Organic contamination poses significant environmental challenges, particularly at low concentrations where traditional activated sludge processes are ineffective. Alternative methods like adsorption, advanced oxidation processes (AOPs), membrane filtration, and ion exchange are employed. Heterogeneous catalytic wet peroxide oxidation (CWPO) is gaining traction due to its operational simplicity and low cost. Nanoscale zero-valent iron (NZVI) is a promising heterogeneous catalyst because of its low cost, environmental friendliness, and high efficiency. However, challenges remain, including surface oxidation, difficulty in separation, decreased catalytic efficiency, NZVI loss, and secondary pollution. Immobilizing NZVI onto stable porous substrates, such as polymeric fiber films, offers a solution. Polymeric films offer several advantages: corrosion resistance, excellent mechanical strength, ease of modification, good ductility, high voidage, and abundant π electrons for strong electrostatic binding of iron nanoparticles. Electrospinning is an effective technique for fabricating polymer fiber films with controllable structural characteristics. Traditional methods for combining metal-based materials and films often involve multiple steps, resulting in uneven active component distribution, decreased catalytic efficiency, low mechanical strength, and polymer structure damage. Weak interactions between nanoparticles and fibers cause nanoparticle leakage, rapid deactivation, and secondary contamination. This study aims to develop a stable and efficient water treatment material by integrating NZVI and a porous PAN fiber film using an innovative one-step cryogenic auxiliary electrospinning method to improve catalytic activity and stability, addressing the limitations of traditional approaches.

The literature extensively covers the use of zero-valent iron (ZVI) for the degradation of organic pollutants. Studies highlight the effectiveness of ZVI in various applications, including the reductive removal of nitrobenzene [1] and the catalytic wet peroxide oxidation of organic compounds [2, 3]. However, the inherent limitations of ZVI, such as easy oxidation and difficulty in separation, have led to research focusing on improving its stability and efficiency. Loading ZVI onto stable supports, particularly polymeric nanofibers, is a common strategy. Research demonstrates the benefits of incorporating ZVI into polymeric nanofibers for enhanced catalytic activity and stability compared to inorganic catalysts [14-16]. Electrospinning has emerged as a preferred method for synthesizing these nanofiber-based catalysts, offering precise control over the morphology and properties of the resultant material [10-12]. Despite the advantages, traditional methods for incorporating ZVI often involve multiple steps, leading to problems such as uneven distribution of active components and poor stability [13,14]. This study aims to address these challenges by employing a novel, one-step method for fabricating a highly stable and efficient ZVI-based catalyst.

This study employed a novel one-step cryogenic auxiliary electrospinning method to synthesize a NZVI-incorporated porous PAN fiber film (Fe-PAN). The process involved dissolving PAN and TOAB in DMF, adding NZVI, stirring, and ultrasonication to create a homogeneous suspension. This suspension was then electrospun using a cryogenic acceptor filled with liquid nitrogen, which induced thermally induced phase separation (TIPS) and the formation of a porous structure. A control sample, C-Fe-PAN, was prepared using conventional electrospinning without the cryogenic acceptor. The Fe-PAN film was characterized using SEM, TEM, EDS, AFM, N2 adsorption-desorption isotherms, contact angle measurements, and water flux tests. The CWPO reaction was optimized using a three-level Box-Behnken design, examining the effects of pH, temperature, and H2O2 concentration on methylene blue (MB) degradation efficiency. The reaction mechanism was investigated using radical scavengers (IPA and p-BQ), and the catalyst structural evolution was monitored using XRD, FTIR, and XPS after multiple cycling runs. HPLC was used to analyze MB mineralization. The degradation kinetics were analyzed using first- and second-order kinetic models. Control experiments were conducted with pure PAN and C-Fe-PAN films to assess the contribution of NZVI and the porous structure to the degradation efficiency. Additional experiments used methyl orange and methyl blue to assess the applicability of Fe-PAN to different types of dyes.

The cryogenic auxiliary electrospinning method successfully produced a highly porous Fe-PAN film with a 3D network structure. The film exhibited a high surface area (1.48 m²/g) and a unique nano-mesoporous structure. The NZVI was evenly dispersed within the PAN fibers, as confirmed by TEM and EDS mapping. The Fe-PAN film demonstrated high catalytic activity, achieving >95% MB removal in just 4 minutes under optimal conditions (pH 2.8, 56 °C, 4.2 mmol/L H2O2). The Box-Behnken design showed pH to be the most significant factor affecting MB degradation. The degradation process followed second-order kinetics, and radical scavenging experiments confirmed the involvement of ·OH and O2 radicals, with O2 playing a more dominant role. The Fe-PAN film exhibited excellent reusability, with complete MB degradation in each cycling run, although the reaction time increased with reuse due to pore blockage and NZVI oxidation. The water flux of the Fe-PAN film was significantly higher than that of reported catalytic films, indicating excellent water permeation ability. XPS analysis revealed that the C≡N groups in PAN, which contribute to enhanced electron transfer, were gradually replaced by oxygen-containing groups during the reaction, leading to a decrease in catalytic activity over time. Despite this structural change, the Fe-PAN film maintained superior performance compared to reported catalytic films. Iron leakage was minimal, less than 2 mg/L even after long-term use.

The high catalytic activity and stability of the Fe-PAN film are attributed to the synergistic interaction between NZVI and the porous PAN fiber structure. The cryogenic electrospinning method allowed for the creation of a highly porous structure which facilitates mass transfer, while the strong interaction between NZVI and the PAN fibers prevents NZVI leaching and improves stability. The second-order reaction kinetics suggest that the availability of active sites is the rate-limiting step. The results highlight the importance of the porous structure and the interaction between NZVI and the PAN fiber in enhancing catalytic efficiency and reusability. The gradual decrease in catalytic activity with reuse is mainly due to pore blockage by reaction byproducts and the oxidation of NZVI. The findings suggest that further research could focus on developing strategies to mitigate pore blockage and NZVI oxidation, such as surface modifications or the addition of protective layers. The study shows that the Fe-PAN film holds promise for practical applications in water treatment due to its high efficiency, stability, and reusability.

This study successfully developed a highly efficient and stable Fe-PAN catalytic film for organic matter degradation using a novel one-step cryogenic auxiliary electrospinning method. The Fe-PAN film demonstrated superior catalytic activity, reusability, and water flux compared to existing catalytic films. The findings highlight the synergistic effect of the porous structure and the NZVI-PAN interaction. Future research should focus on improving long-term stability by addressing pore blockage and NZVI oxidation, potentially through surface modification or integrating self-cleaning mechanisms. The Fe-PAN film holds significant potential for industrial water treatment applications.

The study focused primarily on methylene blue degradation. While additional experiments with methyl orange and methyl blue showed similar success, further research is needed to assess the efficacy of Fe-PAN against a wider range of organic pollutants and under different water matrix conditions. The long-term stability and reusability of the Fe-PAN film were influenced by pore blockage and NZVI oxidation. Further research could explore strategies to minimize these effects to enhance the catalyst's lifetime and overall performance. The study used a batch reactor; scaling up to continuous flow systems and evaluating the film's performance under real-world conditions would be beneficial.